Image display device comprising an auxiliary light source

ABSTRACT

In an image display device comprising a photoconductive layer for collecting an incident radiation image and a field-strengthsensitive medium affected thereby, which medium in turn modulates an auxiliary light beam, the photoconductive layer has to be effectively screened from the auxiliary light. According to the invention an intermediate layer is used for this purpose which consists of a semiconductor whose energy gap is a few tenths ev. smaller than the energy gap of the photoconductive material. This is fulfilled by using copper-activated CdSe for the photoconductor and a semiconductor of the group CdTe, GeS and Sb2Se3 and mixtures thereof for the intermediate layer. The latter two materials have the advantage that they can be vapordeposited on a substrate which is at room temperature. The vapordeposited substances are thus prevented from producing intermediate levels in the photoconductor, in which case the long-wave portion of the auxiliary light passing through the intermediate layer might release charge carriers in the photoconductor. A layer consisting of 3 parts by weight of Sb2Se3 and 1 part by weight of GeS satisfies in particular the relevant requirements owing to the fine crystalline form of a layer of this material when it is vapor-deposited on the photoconductive layer which is at room temperature.

I United States Patent Koelmans [54] IMAGE DISPLAY DEVICE COMPRISING AN AUXILIARY LIGHT SOURCE [72] Inventor: Hein Koelmans, Emmasingel, Eindhoven,

Netherlands [73] Assignee: U.S. Philips Corporation, New York, NY. [22] Filed: Mar. 11, 1970 [21] Appl. No.: 18,675

Primary Examiner-Walter Stolwein Assistant ExaminerD. C. Nelms AttorneyF rank R. Trifari Jan. 25, 1972 57 ABSTRACT In an image display device comprising a photoconductive layer for collecting an incident radiation image and a fieldstrength-sensitive medium affected thereby, which medium in turn modulates an auxiliary light beam, the photoconductive layer has to be effectively screened from the auxiliary light. According to the invention an intermediate layer is used for this purpose which consists of a semiconductor whose energy gap is a few tenths ev. smaller than the energy gap of the photoconductive material. This is fulfilled by using copper-activated CdSe for the photoconductor and a semiconductor of the group CdTe, GeS and Sb Se and mixtures thereof for the intermediate layer. The latter two materials have the advantage that they can be vapor-deposited on a substrate which is at room temperature. The vapor-deposited substances are thus prevented from producing intermediate levels in the photoconductor, in which case the long-wave portion of the auxiliary light passing through the intermediate layer might release charge carriers in the photoconductor. A layer consisting of 3 parts by weight of Sb Se and 1 part by weight of GeS satisfies in particular the relevant requirements owing to the fine crystalline form ofa layer ofthis material when it is vapordeposited on the photoconductive layer which is at room temperature.

5 Claims, 3 Drawing Figures PAYENTEDJMSWZ 34638.02.

' sum; or 2 INVENTOR. HEIN KOELMANS 3 ,63 8 ,027 1 2 IMAGE DISPLAY DEVICE COMPRISING AN AUXILIARY Hereinafter, a few embodiments of an image display device LIGHT SOURCE The invention relates to an image display device comprising a photoconductive layer having at least one electrode for capturing a radiation image projected thereon, there being provided on the side of the photoconductive layer remote from the image-capturing side a field-strength-sensitive medium having a transparent electrode and locally modulating an auxiliary light beam incident thereon in dependence upon the local electric field strength.

An example of such an image display device is given by the solid-state system sold under the trademark -eidophore. In this device, an image light beam striking a photoconductive layer produces an electric charge pattern corresponding to the brightness distribution in the image. This charge pattern results in a locally varying field strength which acts upon a free surface of a field-strength-sensitive medium. Thus, a projection light beam emanating from the other side than the image light and being incident to this medium is modulated in the reflection direction. By using a Schlieren optical system image, information corresponding to the image information of the image light incident on the photoconductive layer is obtained in the projection light beam and can be visualized on a collecting screen. This requires a high light intensification between the image light and the projection light.

Such an image display device is described in an article in Zeitschrift fiir angewandte Mathematik und Physik I8, 1967, pages 3l to 57.

In the image display device described therein, light of the projection light beam may reach, for example, as stray light, the photoconductor so that the contrast in the image gets more or less lost, unless the intensity of the projection light is kept low, but in this case only slight light intensification is obtained.

The invention has for its object to provide an image display device in which the photoconductive layer is satisfactorily screened from the projection light without adversely affecting the transmission of the potential pattern from the photoconductor to the field-strength-sensitive medium.

According to the invention, an image display device of the kind set forth is characterized in that between the photoconductive layer and the field-strength-sensitive medium a lightscreening intermediate layer is provided, which is mainly made of semiconductor material which, like the material of the photoconductive layer, has a direct junction between the valence band and the conduction band, the energy gap between the material of the intermediate layer being so much smaller than that of the material of the photoconductive layer that for radiation corresponding to the direct band transition of the photoconductor the intermediate layer exhibits high absorption. The energy gap of the material of the intermediate layer is preferably at the most about 0.2 ev. smaller than the energy gap of the photoconductor.

Direct band-to-band transitions have a comparatively high absorption coefficient so that a thin intermediate layer pro- .vides adequate screening. Because the material of the photoconductive layer has a larger energy gap than the material of the intermediate layer, the light passing through the latter layer cannot be absorbed by band-to-band transitions in the photoconductive layer. For absorption via intermediate levels, if any, in the photoconductor, the coefficient is comparatively low and this absorption may be kept low by providing a thin photoconductive layer. In order to prevent the intermediate layer from affecting the transmission of the potential pattern from the photoconductive layer to the fieldstrength-sensitive medium, this intermediate layer not only has to be thin but also be of a material having a resistivity at least equal to the resistivity of the unexposed photoconductor material.

By using an intermediate layer according to the invention a considerable light intensification can be obtained between the image light and the projection light without loss of image contrast.

according to the invention will be described with reference to the drawing, in which FIG. 1 is a schematic sectional view of an image display device comprising an intermediate layer and FIG. 2 is an isometric projection of the layer portion in the reverse arrangement and FIG. 3 shows a. further embodiment of an image display device according to the invention.

The image display device shown in FIGS. I and 2 comprises a first portion 1 and a second portion 2, separated from each other by an intermediate space 3. The first portion 1 serves to convert the image information of an incident image light beam 4 into an electric charge pattern, which produces in the second portion 2 the modulation of a projection light beam 5 striking the side of the portion 2 remote from the portion 1 in accordance with the image information of the radiation or image light beam 4.

For this purpose the portion 1 shown in FIG. 1 comprises from below in order of succession and shown in FIG. 2 in the reverse; a transparent support 6, an opaque line pattern 7, an electrically insulating plate 8, a raster system 11 formed by two interengaging, electrically conductive combs 9 and 10, a photoconductive layer 12 and a screening intermediate layer 13. In the portion 2 the intermediate space 3 is followed by a medium 14 deformable locally by an electric field strength, a transparent electric conductor 15 and a transparent support 16. By arranging a prism 17 on the support 16 the medium 14 to be modulated is incorporated in a Schlieren optical system which comprises in addition a light source 18 producing the projection beam 5, a first diaphragm system 19, a second diaphragm system 20, a lens system 21 and a collecting screen 22 capturing the modulated projection light beam. This screen is shown in FIG. I for the sake of simplicity too near the lens system 21 and on too small a scale. The prism 17, which may be replaced by a lens system, ensures that the incident projection light beam strikes the free surface 23 of the deformable medium 14 facing the intermediate space 3 at such an angle that total reflection occurs. The diaphragm systems 19 and 20 are relatively arranged so that the projection light passing through the system 19 and reflected from a surface 23 not deformed by a charge pattern is captured by the opaque parts of the diaphragm system 20 so that the collecting screen 2 does not capture light.

For explaining further the functions of the parts of the portions 1 and 2, these parts are illustrated in FIG. 2 in an isometric projection. In FIG. 2, the image is shown as being projected from above on the portion 1. In one of the known methods of producing a potential pattern, a direct-voltage source 24 is connected between the combs 9 and I0 and a direct-voltage source 25 between a central tapping of voltage source 24 and the electrode 15, which may consist of a vapor-deposited tin oxide layer. Between the raster system 11 and the electrode 15 there is thus produced a wave-shaped potential pattern corresponding to the line pattern of the raster system 11, while the surface 23 of the deformable medium 14 is deformed accordingly. Owing to this wave-shaped pattern the projection light is locally changed in direction upon reflection, but by arranging the direction of the lines of the raster system 11 at right angles to the diaphragm systems 19 and 20 said change of direction coincides with the direction of the gaps of the systems 19 and 20 so that also in this case no projection light can reach the screen 22. An image light beam passing through the raster system 11 to the photoconductive layer 12 is capable of locally intensifying or weakening this wave pattern, it is true, but even then the screen 2 does not receive light. This situation is not changed until a line pattern 7 of opaque strips whose direction is at an angle to the direction of the raster system 11 is present in the path of the image light. Incident image light will have a torsional effect on the direction of the waves of the wave pattern because the light falling across it in the form of oblique strips affects in the same sense the electrical conductivity of the photoconductive layer and hence the potential pattern. Thus, the projection light is deflected transversely and this deflected light will pass through the diaphragm system 20 and produce an image on the collecting screen. This arrangement, further variants of which are possible, ensures that the image information of the image on the photoconductive layer is reproduced on the collecting screen. In order to avoid disturbances in this transmission, the line pattern 7, preferably formed by vapor-deposited metal strips, has to be shaded satisfactorily on the raster system 11 and the photoconductive layer 12, but it has to be electrically insulated from the combs 9 and 10. For this purpose it is preferred to provide a glass plate 8 of about 100p. thick on one side with the line pattern 7 and on the other side with the raster system 11 by vapor deposition, the photoconductive layer 12 being directly applied thereto by vapor deposition. The field strength pattern is affected by light incidence on the photoconductive layer and extends between the raster system 11 and the electrode 15. The distance between the photoconductive layer 12 and the electrode 15 has to be as small as possible for obtaining a high sensitivity, i.e., a high-field strength between both of them. Consequently, the intermediate layer 13, the intermediate space 3 and the deformable layer 14 have to be thin. The deformable layer is formed by a layer of an oil having a fairly low-electrical resistance, for example, polymerized silicon oil so that the field strength thereon is low. Therefore, especially the intermediate space 3 and the screening layer 13 have to be thin. A minimum value of the thickness of the intermediate space 3 is determined by the risk of relative contact between the layers 13 and 14 and it is about 2011.. The thickness of the intermediate layer 13, which is preferably deposited directly on the photoconductive layer 12 from the vapor phase, need be not more than a few microns when the invention is used.

In the present case, the photoconductive layer 12 consists a copper-activated CdSe, the thickness being at the most 1p. By doping, for example, with copper, the photoconductor may be given a comparatively low-dark resistance of about Ohmcm. This semiconductor material CdSe has an energy gap of 1.8 ev. This material is chosen in order to satisfy in a simple manner the requirements of an appropriate screening intermediate layer 13. This intermediate layer consists of CdTe, GeS or Sb Se or mixtures thereof. These semiconductor materials having energy gaps of 1.5 ev., 1.7'ev. and 1.4 ev. respectively are deposited from the vapor phase to a thickness of not more than a few microns on the photoconductive layer 12 and constitute satisfactorily adhering, uniform layers. Particularly satisfying is a layer of a mixture of about 3 parts by weight of Sb Se and 1 one part by weight of GeS. The material of the intermediate layer 13 is vapor-deposited on the photoconductor 12 at room temperature. The material of the intermediate layer 13 is thus prevented from forming intermediate levels in the photoconductor during the vapor deposition which levels might absorb the long-wave light passing the intermediate layer 13. By said vapor deposition of the intermediate layer 13 the latter has a fine crystalline structure, the electrical resistance being formed by the crystal transitions. As long as the crystals are small as compared with the raster pitch of the system, the comparatively high-electric conductivity inside the crystals of the intermediate layer 13 is not a source of trouble. With a raster pitch of 5011. is satisfied with certainty because the crystals are in practice not larger than about 1 a. The quality of the intermediate layer 13 is found to be favored by depositing the material from the vapor phase in a nonreducing atmosphere.

FIG. 3 shows a further embodiment of an image display device in accordance with the invention. This device comprises between two transparent supports 30 and 31, viewed in the Figure from left to right; a transparent electrode 32, a photoconductive layer 33 of CdSe, a screening intermediate layer 34, consisting like the said intermediate layer 13, preferably of a mixture of Sb Se and GeS, mosaic layer 35, for example, of crackled aluminum foil, a field-strength-sensitive medium 36 consisting in this case of a nematic, mesoporphrc compound or a nematic, liquid crystal compound, whose light-reflection or light-transmission properties are locally affected by the electric field strength, and finally a second transparent electric conductor 37, which is connected by way of an electric voltage source 38 to the electrode 32.

An image light beam 39 incident from the left to the photoconductive layer 33, produces in accordance with its image information local electric field-strength variations in the nematic layer 36. The corresponding image information in the form of light-transmission or light-reflection variations is employed for modulating a projection light beam 40, which reaches the nematic layer from the other side than the image light beam 39.

For this purpose, the assembly of layers is accommodated in a reproducing device comprising furthermore a lamp 411 for producing the projection light, a semitransparent mirror 42 and a lens system 43 with a collecting screen 44 for capturing the image.

If the light-reflection variation of the nematic layer 36 is employed for modulating the projection beam, a Schlieren optical display system can be used, because essentially a stray effect is then concerned. If on the contrary the absorption variation is used, the light reflected from the aluminum foil 35 produces by way of the semitransparent mirror 42 and the lens system 43 a direct reproduction on the collecting screen 44.

What is claimed is:

1. An image display device comprising a layer of photoconductive material, an electrode system on said photoconductive layer for producing a charge distribution as determined by an image impinging on said layer, a field-strength-sensitive medium, a light-pervious electrode on said medium, and a light screening intermediate layer between said photoconductive layer and said medium, said photoconductive layer, said intermediate layer, and said medium being in confronting relationship, said intermediate layer comprising a semiconductor material, said semiconductor material and said photoconductive material each having a direct band transition between valence and conduction bands, said semiconductor material having an energy gap so much smaller than the energy gap of said photoconductive layer material whereby said intermediate layer is strongly absorptive for radiation corresponding to said direct band transition of said photoconductive layer material.

2. An image display device as claimed in claim 1, wherein the energy gap of the material of the intermediate layer is at the most about 0.2 ev. smaller than the energy gap of the photoconductive layer.

3. An image display device as claimed in claim 1 wherein the photoconductive layer is formed by a photoconductor of copper-activated CdSe and the intermediate layer comprises of a fine crystalline layer material selected from a semiconductor of the group consisting of CdTe, GeS and Sb Se 4. An image display device as claimed in claim 1 wherein the photoconductive layer is formed by a layer of CdSe of a thickness of at the most lu vapor-deposited on an electroded, transparent support and the intermediate layer comprises a layer of GeS or Sb Se of a thickness of at the most a few microns, vapor-deposited on said photoconductive layer.

5. An image display device as claimed in claim 1 wherein the intermediate layer comprises a mixture of about 3 parts by weight of Sb Se and 1 one parts by weight of GeS. 

2. An image display device as claimed in claim 1, wherein the energy gap of the material of the intermediate layer is at the most about 0.2 ev. smaller than the energy gap of the photoconductive layer.
 3. An image display device as claimed in claim 1 wherein the photoconductive layer is formed by a photoconductor of copper-activated CdSe and the intermediate layer comprises of a fine crystalline layer material selected from a semiconductor of the group consisting of CdTe, GeS and Sb2Se3.
 4. An image display device as claimed in claim 1 wherein the photoconductive layer is formed by a layer of CdSe of a thickness of at the most 1 Mu vapor-deposited on an electroded, transparent support and the intermediate layer comprises a layer of GeS or Sb2Se3 of a thickness of at the most a few microns, vapor-deposited on said photoconductive layer.
 5. An image display device as claimed in claim 1 wherein the intermediate layer comprises a mixture of about 3 parts by weight of Sb2Se3 and 1 one parts by weight of GeS. 